His was one of the saddest of all showbusiness stories, long before anyone thought of remaking Sleuth, and asking Kenneth Branagh to direct it. The original - both the stage play and the movie - was a study in camp nastiness, and a stupefying chess match etched in homosexual malice. The veteran American director Joseph L Mankiewicz handled it with sublime innocence of the subtext. That didn't matter. Laurence Olivier and Michael Caine still did it as a kind of back-to-back, blindfolded tango for a Women's Institute coach party from Worthing. Gay stuff was so much more novel then, and Olivier spent much of his career on the principle that he wasn't quite gay - just intrigued by it.

Had he still been around, there was an impresario cruelty in Olivier sufficient to imagine casting head prefect Caine with that new boy - Jude Law. But it took hysterical, wrong-headed ingenuity to get hold of a fragile Harold Pinter to do the screenplay. Just think of it - Branagh, Caine, Law and Pinter - the four most smug and threadbare talents in the English arts? Or do we have other contenders? The strange notion that took hold in Caine's mind - that he is a wine-taster among actors and a fabulous minimalist - has never gone one step towards disguising the coarseness of his personality. As for Law, it is hard to think of so promising, sparrow-like an actor who has been reduced to the status of Howard Hughes' Spruce Goose - an aircraft that never takes flight. In the last five years, in general, ever since his brilliant Dickie Greenleaf in The Talented Mr Ripley, Law has sent off copious alarm signals that amount to "Don't hire me!"

But Branagh is something else. Go back to the late 1980s, when he was still in his 20s, and it was generally reckoned that Ken and Emma (Thompson) were the new generation's answer to Larry and Vivien - but with extra wit, muscle, and democratic urges. In his own Henry V (1989), Branagh showed what a convenient blind eye Olivier had turned to war and its real brutalities. As an actor, Branagh was unquenchable - for television, he did anything and everything: Jimmy Porter in a revival of Look Back in Anger; Oswald in Ghosts; a superb, self-effacing Guy Pringle in Fortunes of War, and a remake of O'Neill's Strange Interlude in which he starred with Glenda Jackson. Usually, he was with Emma, and it may be that her cool judgment was vital to the directions he was seeking.

Alas, he left her and settled for being an international director, and the work deteriorated - Dead Again, Peter's Friends, Much Ado About Nothing, the quite dreadful Frankenstein, a Hamlet that was nowhere near as good as Olivier's, and so it goes, all the way to a very strange rendering of The Magic Flute and the dire Sleuth.

Of course, there was no pressing need for Branagh to be a director. It would have been enough for him to advance as an actor - real with a southern accent in Altman's The Gingerbread Man, exhausted and drained in Woody Allen's Celebrity (1999), Wild Wild West (1999), and a little Harry Pottering.

You might be excused for giving up on Branagh, and maybe that is how you missed Conspiracy (2001), one of the most chilling films in recent years, directed by Frank Pierson. It is a record of the Wannsee Conference of 1942, based on authentic transcripts, in which Nazi civil servants planned the final solution. Branagh played Reinhard Heydrich, the mastermind behind the conference, and it is quite simply a great performance, as well as the remaining, nagging proof that Branagh, the actor, still possesses an astonishing authority.

It is unaccountable that Branagh, still under 50, has sunk so low and felt bound to accept the invitation to remake Sleuth. Perhaps he was over-praised once. Perhaps he was too willing to believe all his notices. It is a long way back, but half an hour of Conspiracy will convince you - this is a real firebrand of an actor with an uncommon sense of wickedness.